The present invention relates to exploration for sources of hydrocarbon fuel and particularly to enhancing the safety of oil and gas well drilling procedures. More specifically, this invention is directed to apparatus and methods for detection of the infusion of gas into a borehole and especially to apparatus and methods for a gas infusion detection system which detects gas influx in the marine riser of an offshore drilling rig.
In the drilling of oil and gas wells, drilling safety and efficiency are extremely important. One safety problem relates to what is known as a "blowout". A zone of high geopressure, contained by cap rock, will occasionally be unknowingly encountered during drilling. If this pressure exceeds the hydrostatic pressure exerted by the drilling mud, and the formation has sufficient permeability to allow fluid flow, then the formation fluid will displace the drilling mud. This is referred to as a "kick"; and if unchecked will cause what is known as a "blowout" condition. One borehole condition which the driller desires to monitor, in order to ensure against "blowout" is gas influx.
Various techniques have previously been proposed, and in some cases implemented for measuring gas infusion into a borehole including sensing the borehole annulus pressure, sensing the pressure differential between the interior of the drill string and the annulus, measuring the velocity of sound in the drilling mud, measuring the resistivity of the drilling mud and various other tests based upon attempts to measure the pressure of the formation through which the drill string is penetrating or has penetrated. These previously proposed gas detection techniques, and particularly those based upon pressure measurements, all have deficiencies which severely limit their usefulness. An improved borehole fluid influx detection system is described in U.S. Pat. Nos. 4,733,232 and 4,733,233, both of which are assigned to the assignee hereof and incorporated herein by reference. In accordance with the methods of these prior patents, the pressure in the annulus between the standpipe (drill pipe or string) and wall of the well is monitored at the surface. Frequency or amplitude modulation of the mud flow in the standpipe by a coherent energy source at a point near the drill bit will result in the mud flow in the annulus containing information in the form of reflections of the modulation of the flow in the standpipe. Pressure monitoring of the mud flow in the annulus at the surface thus results in the detection of the reflected information resulting from modulation of the column of drilling mud in the drill string (standpipe). The pressure variations detected in the annulus are compared to pressure variations detected in the standpipe. A significant change in phase and/or amplitude ratio between the standpipe and annulus pressure variations, particularly a change in phase and/or amplitude ratio which constitutes a significant deviation from a previously established history, will indicate that there is a fluid influx into the annulus since fluid, for example gas, flowing into the drilling mud will produce attenuation of the modulated information and/or will affect the transmission velocity.
While well suited for its intended purposes, the method of U.S. Pat. Nos. 4,733,232 and 4,733,233 is not adapted for measuring gas influx (e.g. riser gas) when no drill string is present in the borehole. This is a particularly serious problem in offshore oil rigs where there is a need to measure gas influx whether a drill string is present in the borehole or has been withdrawn.